An acid herbicide used in weed killer banned from retail sales since 2008 is turning up in Selwyn compost.

Clopyralid is found in several over the counter herbicides that were available before 2008, including Versatill, No Lawn Weeds Clover and Prickle, Cardo, Radiate, Archer, Tango, Contest Multiple, Void, Clout, Clopyd 300, Pirate 300, Agpro, and Cloralid 300.

Selwyn District Council Solid Waste Manager Gavin Sole says people are probably still using the weed killer preparations on their lawns, unaware of the toxic downstream effects.

Clopyralid continues to have an effect on lawns for about three months but is toxic for any composts. The SDC is asking gardeners not to put lawn clippings that have been sprayed with any clopyralid preparations in their organics bins. They also need to be careful about using the lawn clippings on their own gardens.

"We've recently screened 1800m3 of compost at the Pines Resource Recovery Centre and discovered higher than recommended levels of this toxic chemical, which means we will need to spend time and money to continue to treat the compost until it is safe for sale. It makes a lot more expensive per tonne to treat the incoming organics if we have contaminants such as Clopyralid in the grass clippings" Mr Sole says.

Gardeners who are still using the pre-2008 preparations can either put their grass clippings in their general rubbish bin, spread them under trees on their own properties, or drop off the old preparations at the Pines facility, where disposal will be arranged. More modern herbicides do not contain the toxic chemical.

Retailers in the district also need to be aware of the banned products as these should not be available for sale to the general public, Mr Sole says.